busybox: use a single udhcpc script, with or without avahi-autoipd

We're currently using two different udhcpc scripts, one in the busybox
package and another in the avahi one, which calls avahi-autoipd on
dhcp failures.

The avahi one actually only does something differently from the default
if avahi-autoipd is available, so let's just always use this one instead
of the complicated logic about writing the file if not present /
overwriting it afterwards.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>